1. Field of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. The lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays, and other devices involving fine structures. In a conventional lithographic apparatus, a patterning means, which is alternatively referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device), and this pattern can be imaged onto a target portion (e.g., comprising part of one or several dies) on a substrate (e.g., a silicon wafer or glass plate) that has a layer of radiation-sensitive material (e.g., resist). Instead of a mask, the patterning means can comprise an array of individually controllable elements that generate the circuit pattern.
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning” direction), while synchronously scanning the substrate parallel or anti-parallel to this direction.
In a maskless lithographic apparatus, a polarizing beam splitter is generally used to project the radiation beam onto the individually controllable elements. The radiation beam is projected through the beam splitter twice and a quarter wave plate is used to change the polarization of the radiation beam after first transmission through the beam splitter and before the second transmission through the beam splitter. Use of the polarization to control the direction of the radiation beam means that the cross section of the radiation beam has a uniform polarization, and thus different polarizations cannot be used to create different effects during the exposure. Also, beam splitters are inefficient and can be difficult to thermally control.
A non-polarizing beam splitter, with a half silvered mirror, can be used instead of a polarizing beam splitter to avoid polarization issues, but with two passes through such a device about 75% or more of the radiation is lost, substantially reducing throughput.
Therefore, what is needed is a system and method including a lithographic apparatus in which beam splitters are no longer necessary.